This invention relates to an electronic liquid level gauge for measuring the level of a liquid contained in a vessel and, more particularly, to such an electronic liquid level gauge employing an electrode pair having two electrode plates facing in spaced-parallel relation to each other and immersed in the liquid, the electrode pair having an electrostatic capacity determined by the liquid level.
It has theretofore been proposed to measure the level of a liquid contained in a vessel by employing an electrode pair having two electrode plates faced in spaced-parallel relation to each other and immersed in the liquid, the electrode pair having an electrostatic capacity determined by the liquid level. Assuming now that the specific inductive capacity of the liquid is .epsilon..sub.G, the length of the electrode pair is L, and the electrode pair has a portion immersed in the liquid, the length of which portion is l, the electrostatic capacity C of the electrode pair is defined by EQU C=K{(L-l)+.epsilon..sub.G l}=K{(.epsilon..sub.G -1)l+L} (1)
wherein K is a constant determined by the shape of the electrode pair and the distance between the electrode plates. Solving for l we have EQU l=(C/K-L)/(.epsilon..sub.G -1)=A(C-B) (2)
wherein A=1/{K(.epsilon..sub.G -1)} and B=LK.
It is the conventional practice to associate the electrode pair with an oscillator which generates oscillator pulses at a repetition period T directly proportional to the electrostatic capacity C of the electrode pair. The oscillator pulse repetition period T, which is represented by DC, is detected by a detector. The detector output is coupled to a subtractor which subtracts a constant B' from the oscillator pulse repetition period D.multidot.C. The resulting difference (DC-B') is multiplied by a constant A' in a multiplier. The resulting product A'.times.(DC-B') can be made to correspond to the liquid level l by suitably selecting the constants A', B' and D to satisfy equation (2). However, such conventional practice utilizes a binary subtractor and a binary multiplier and requires complex hardware and expensive microcomputer for accurate liquid level measurements.
In some instances where it is required to measure the level of liquid such as, for example, petrol having its specific inductive capacity changed as a function of temperature or by mixing therein petroleum economizer or other additives, another electrode pair has been employed in addition to the former electrode pair. The latter electrode pair has two electrode plates facing in spaced-parallel relation to each other and immersed in the liquid, the electrode pair having an electrostatic capacity determined by the specific inductive capacity of the liquid. Assuming that the former electrode pair has a length L, an area S.sub.1, and a portion immersed in the liquid, the length of which portion is l, that the distance between the electrode plates of the former electrode pair is d.sub.1, and that the specifc inductive capacity of the liquid is .epsilon..sub.r and the dielectric constant of air is .epsilon..sub.o, the electrostatic capacity C.sub.1 of the former electrode pair is given by ##EQU1## Solving for l we have ##EQU2## wherein A" is a constant represented by S.sub.1 .multidot..epsilon..sub.o /L.multidot.d.sub.1 and C.sub.o =A".multidot.L=S.sub.1 .multidot..epsilon..sub.o /d.sub.1 is the electrostatic capacity of the former electrode pair when the vessel is emptied.
Assuming that the latter electrode pair has an area S.sub.2 and the distance between the electrode plates is d.sub.2, the electrostatic capacity C.sub.2 of the latter electrode pair is given by EQU C.sub.2 =.epsilon..sub.o .multidot..epsilon..sub.r .multidot.S.sub.2 /d.sub.2 ( 5)
Solving for .epsilon..sub.r we have ##EQU3## and equation (4) becomes ##EQU4## wherein K is a constant represented by (d.sub.2 .multidot.S.sub.1)/(d.sub.1 .multidot.S.sub.2).
It is the conventional practice to associate the former electrode pair with an oscillator which generates oscillator pulses at a repetition period T.sub.1 directly proportional to the electrostatic capacity C.sub.1 of the former electrode pair. The oscillator pulse repetition period T.sub.1, which is represented by B.sub.1 .multidot.C.sub.1, is converted into a binary number by a repetition period detector. The detector output is coupled to a subtractor which subtracts a constant B.sub.1 .multidot.C.sub.1 from the repetition rate binary value and provides a difference B.sub.1 .multidot.(C.sub.1 -C.sub.o).
The latter electrode pair is associated with an oscillator which generates oscillator pulses at a repetition period T.sub.2 directly proportional to the electrostatic capacity C.sub.2 of the latter electrode pair. The oscillator pulse repetition period T.sub.2, which is represented by B.sub.2 .multidot.C.sub.2, is converted into a binary number by a repetition period detector. The detector output is coupled to a multiplier which multiplies a constant K by the repetition period binary number B.sub.2 .multidot.C.sub.2. The multiplier output is coupled to a subtractor which subtracts a constant B.sub.2 .multidot.C.sub.o from the resulting product K.multidot.B.sub.2 .multidot.C.sub.2 and provides a difference B.sub.2 .multidot.(K.multidot.C.sub.2 -C.sub.o). The resulting difference B.sub.1 .multidot.(K.multidot.C.sub.2 -C.sub.o) is divided by the difference B.sub.2 .multidot.(K.multidot.C.sub.2 -C.sub.o) in a divider which provides a value B.sub.1 .multidot.(C.sub.1 -C.sub.o)/B.sub.2 .multidot.(K.multidot.C.sub.2 -C.sub.o). The divider output is coupled to a multiplier which multiplies the value B.sub.1 (C.sub.1 -C.sub.o)/B.sub.2 .multidot.(K.multidot.C.sub.2 -C.sub.o) by a constant L.multidot.B.sub.2 /B.sub.1 and provides a product L.multidot.(C.sub.1 -C.sub.o)/(K.multidot.C.sub.2 -C.sub.o), which agrees with the liquid level l represented by equation (7). However, such conventional practice utilizes two binary dividers, two binary subtractors, and two binary multipliers and requires complex hardware and expensive microcomputer for accurate liquid level measurements.
This invention provides an improved electronic liquid level gauge which employs a simple and inexpensive electronic circuit and gives correct liquid level measurements within close tolerances.